1. Field of the Invention
The invention relates generally to materials used in polar and semi-polar semiconductor devices and, more particularly, to polarization-induced bulk doping techniques to reduce the series resistance of light emitting diodes (LEDs) and improve lateral current spreading.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. A bias is applied across the doped layers, injecting holes and electrons into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED. A typical high efficiency LED comprises an LED chip mounted to an LED package, wire-bonded to make electrical contacts, and encapsulated by a transparent medium. The efficient extraction of light is a major concern in the fabrication of LEDs.
A useful measure of the efficiency of an LED is the wall-plug efficiency. This is a measure of the electrical-to-optical power conversion. Much effort has been devoted to improving the wall-plug efficiency of LEDs. One way to do this is to decrease the series resistance of the LED which in turn lowers the operating voltage. Each internal component or layer of the LED contributes to the total series resistance. Thus, reducing the resistance of any component or layer would also reduce the total series resistance of the LED. Lowering the resistance of the p- and the n-cladding layers of an LED improves lateral current spreading, especially when the lateral current spreading is entirely due to a semiconductor layer rather than a high conductivity metal contact layer.
A known method for reducing resistance is by bulk doping an electronic material with impurities. With impurity doping the carrier concentration and transport properties are determined by temperature, dopant concentration and scattering mechanisms such as impurity doping and phonon scattering. The carrier mobility is always diminished by the ionized impurity scattering. The carrier concentration is reduced as temperature decreases. These problems led to research in the area of modulation doping which has been shown to improve low temperature carrier mobility in quantum-confined structures by many orders of magnitude.
Recently, group III-nitrides (e.g, AlN, BN, GaN, InN) have emerged as important materials for high-power microwave electronic and LED applications. Crystals such as group-III nitrides, when grown along the [0001] or the [000-1] direction of the wurtzite structure, exhibit large embedded electronic polarization fields owing to the lack of inversion symmetry in the crystal structure. This suggests the existence of a dipole in each unit cell of the crystal. For a homogeneous bulk crystal surface, dipoles inside the crystal cancel and leave net opposite charges on the opposing crystal surface, which is characterized by spontaneous polarization. Dipoles can also be created when a crystal is under strain, characterized by piezoelectric polarization. Both spontaneous polarization and piezoelectric polarization have been exploited for applications in communications, radar, infrared imaging, tunnel junction diodes, high-electron mobility transistors, memories, integrated optics, and in many other fields.
In one of the most popular nitride electronic devices, high-electron mobility transistors (HEMTs), the strong spontaneous and piezoelectric polarization fields in AlGaN and GaN have been used to make nominally undoped two-dimensional electron gases (2DEGs) in AlGaN/GaN heterostructures. [See Mishra et al., United States Patent Application Publication No. US 2006/0231860 A1 (Oct. 19, 2006)]. These devices can yield excellent power and efficiency performance at microwave frequencies.
Research in polarization doping in HEMT devices recently led to the development of three dimensional electron slabs that are usable as bulk doped carriers. This is done by grading the heterojunction of a material system such as AlGaN/GaN over a distance to spread the positive polarization charge into a bulk three-dimensional polarization background charge. The removal of ionized impurity scattering results in higher mobilities and better operation at lower temperatures. Experimental results have shown more than an order of magnitude improvement of carrier mobility at low temperatures for the polarization doped system over comparable donor-doped system. [See Jena et al., Realization of wide electron slabs by polarization bulk doping in graded III-V nitride semiconductor alloys, Applied Physics Letters, Vol. 81, No. 23 (December 2002)].